We propose to continue testing our original hypothesis that virus infections alter the surfaces of host cells causing them to be selectively susceptible to adherence of potentially pathogenic bacteria. We have selected Staphylococcus aureus and human influenza A virus for continue study because staphylococcal pneumonia is a well-documented example of a life-threatening bacterial superinfection occurring in patients with complicated influenza. We propose to extend our previous studies, using a simple cell culture model system, to a much more complex animal model system. We have selected to use ferrets (Mustela putorius furo), which are small carnivores, because influenza in the ferret most resembles the disease in humans. One recent isolate of S. aureus from a patient who died with post-influenza staphylococcal pneumonia, and a human strain of influenza A/Hong Kong/1/68 (H3N2) virus are the primary infectious agents that will be used throughout the in vivo study. Influenza A virus-infection in ferrets will be studied using different doses of the virus and different routes of inoculation (intranasal, aerosol, and natural transmission). Virus-infected or sham-inoculated control ferrets will be sacrificed at specific time intervals; blood, respiratory washes, and respiratory tissues will be examined for virus distribution patterns and infectivity, sialoprotein content of mucin, the presence of nonspecific viral inhibitors, interferon levels, seroconversion, and hematologic changes. Staphylococcal infection in ferrets will also be studied using different doses of the bacteria and different routes of inoculation (intranasal, aerosol, and the 'carrier state') Respiratory tissues will be harvested at various time intervals for quantitative culture to determine the distribution and clearance of bacteria from different sites in the tract. The optimal conditions (dose, route of inoculation, stage of virus infection) for inducing a combined infection with influenza A virus and S. aureus will be determined, and the morbidity and mortality rates will be compared with those obtained with ferrets exposed to each agent alone. Lastly, an in vivo radioassay will be used to establish adherence patterns of radiolabeled bacteria to mucosal surfaces in the respiratory tract, and to determine the factors that affect adherence of bacteria to these surfaces.